why do people say "have a good monday?"

mondays are, by definition, crappy days.
personally for me, mondays mean that i realize that all of those things that i was supposed to do over the weekend, but didn't?
they come back with a vengence.
clawing and ripping.
snarling with their massive fangs.
(just so you know, those things that i'm supposed to do are personified in the form of the abominable snow man)
so mondays are really scary.
so why taunt me by saying "happy monday?"